The present invention relates to a current driver circuit in which data for a magnetic signal from a write current pre-amplifier is written to a magnetic storing device such as a magnetic disk driven by a hard disk drive (HDD) or a floppy disk drive (FDD).
FIG. 9 is a circuit view showing a conventional current driver circuit. In FIG. 9, Tr1 and Tr2 indicate current drivers including P-channel metal oxide semiconductor (PMOS) transistors respectively. Tr3 and Tr4 indicate current receivers including N-channel metal oxide semiconductor (NMOS) transistors respectively. Tr5 to Tr8 indicate NMOS transistors, respectively. WH indicates a write head for producing a magnetic signal from a write current. CS indicates a current source for supplying a reference current Iref. A voltage of a high voltage source is applied to gates of the current receivers Tr3 and Tr4 and the transistor Tr7 through the current source CS so as to turn on the current receivers Tr3 and Tr4 and the transistor Tr7.
FIG. 10 is a timing chart showing an operation of the circuit.
Next, an operation of the conventional current driver circuit will be described below.
The conventional current driver circuit is shown in FIG. 9 as an example of writing data with a magnetic signal onto a magnetic disk driven by an HDD or the like. Each of the current drivers Tr1 and Tr2 functions as a switch. When a gate terminal A of the current driver Tr1 is set to a low level of V11, the current driver Tr1 is turned on. When a gate terminal B of the current driver Tr2 is set to a low level of V11, the current driver Tr2 is turned on. A current mirror circuit includes the current receivers Tr3 and Tr4 and the transistor Tr7. In cases where a high voltage of Vh2 is applied to a gate terminal of the transistor Tr8, the transistor Tr8 is turned on, and the current mirror circuit functions as a current source circuit. When the transistor Tr5 or Tr6 is turned on, the current mirror circuit functions as a current source circuit. That is, a switching of the current source circuit occurs in the current receivers Tr3 and Tr4 according to the on-off of the transistor Tr5 or Tr6.
The gate terminals A and B of the current drivers and gate terminals C and D of the transistor Tr5 or Tr6 are set to levels respectively as shown in the flow chart of FIG. 10. In a time period T1, the current driver Tr1 and the transistor Tr6 are turned on. Therefore, when the current receiver Tr4 is turned on in the switching operation, a write current Iwc flows through the write head WH in a right direction in FIG. 9. Also, in a time period T2, the current driver Tr2 and the transistor Tr5 are turned on. Therefore, when the current receiver Tr3 is turned on in the switching operation, a write current Iwc flows through the write head WH in a left direction in FIG. 9. The write head WH is formed of a coil. Therefore, a magnetic field is induced in the write head WH due to the write current Iwc flowing through the write head WH, the direction of the magnetic field is changed according to the change of the flow direction of the write current, and a magnetic signal is produced. The write head WH is disposed near to a magnetic disk (not shown), and data of the magnetic signal is written to the magnetic disk.
The current drivers Tr1 and Tr2 and the current receivers Tr3 and Tr4 are disposed in the inside of an integrated circuit, and the current drivers Tr1 and Tr2 and the current receivers Tr3 and Tr4 are connected with external devices (for example, the write head WH) through pins of the integrated circuit. Also, the write head WH is connected with pins of the integrated circuit through inter-connectors.
Recently, a data transfer rate of the magnetic signal has been increased more and more in case of the use of the HDD or the like. To write data of the magnetic signal in the magnetic disk at a high transfer rate, it is required to rapidly change the flow direction of the write current Iwc. Therefore, to heighten the data transfer rate, it is required to shorten both a rise time Tr and a fall time Tf of the write current Iwc. Because an operation frequency has been recently increased, current driving performance of the transistors is temporarily increased during a flow direction changing operation of the write current Iwc. Therefore, the rise time Tr and the fall time Tf of the write current Iwc can be shortened. However, in cases where the impedance matching between the write head WH and a group of pins of the integrated circuit connected with the write head WH is not obtained, energy of the current generated in the integrated circuit is not perfectly consumed as the write current Iwc, and energy not consumed is returned to a current supply end as a reflected component. The returning of the reflected component is called an undershoot phenomenon.
FIG. 11 shows a waveform view showing the undershoot phenomenon. In cases where a degree of the xe2x80x9cundershootxe2x80x9d in the phenomenon exceeds a certain level, the magnetic field is not sufficiently induced to write data of the magnetic signal in the magnetic disk. As a result, when the data is read out from the magnetic disk, there is high probability that a reading-out error occurs. Therefore, to heighten the data transfer rate, it is required to solve the problem of the returning of the reflected component, and the impedance matching between the write head WH and the group of pins of the integrated circuit connected with the write head WH is necessary in the current driver circuit.
In the conventional current driver circuit shown in FIG. 9, current flows through the current driver Tr1 or Tr2 functioning as an MOS transistor switch, the write head WH and the line of the current source circuit (that is, a group of the current receiver Tr3 and the transistor Tr5 or a group of the current receiver Tr4 and the transistor Tr6) in that order. An output impedance of the current driver Tr1 or Tr2 is equal to 1/(xcex2xc3x97(Vgsxe2x88x92Vth)), and an input impedance of the current source circuit is equal to 1/(xcexxc3x97Id). Here, Id denotes a drain current. Vgs denotes a difference in voltage between a gate and a source of the current driver Tr1 or Tr2. Vth denotes a threshold voltage of the current driver Tr1 or Tr2. xcex2 and xcex are described later. Though the output impedance of the current driver Tr1 or Tr2 and the input impedance of the current source circuit depend on process and bias conditions, the input impedance of the current source circuit is considerably higher than the output impedance of the switch.
An example of an output impedance of the MOS transistor switch and an example of the input impedance of the current source circuit are described. An output resistance of the MOS transistor switch is expressed by Rout(SF), and an input resistance of the current source circuit is expressed by Rin(CS).
Rout(SF)=1/(xcex2xc3x97(Vgsxe2x88x92Vth))=1/(kxc3x97(W/L)xc3x97(Vgsxe2x88x92Vth))
k=xcexcnxc3x97Cox=xcexcnxc3x97∈ox/tox
Here, W denotes a gate width, L denotes a gate length, xcexcn denotes a mobility of electrons, Cox denotes a fixed capacitance of an oxide film (SiO2), ∈ox denotes a dielectric constant of the oxide film, and tox denotes a thickness of the oxide film. In the 0.25 xcexcm process, xcexcn=0.05 m2/(V*s), ∈ox=34.5 pF/m and tox=6 nm are satisfied. Therefore, k=290 xcexcA /V2 is obtained. In cases where W=500xcexcm and L=0.25 xcexcm are satisfied, W/L=2000 is obtained. When the write current Iwc set to 50 mA, Vgs set to 2.5 V and Vth set to 0.7 V are satisfied, Rout(SF)=1.4xcexa9 is obtained.
Rin(CS)=1/(xcexxc3x97Iwc))
xcex=(1/2L){square root over ( )} (2∈s/(qxc3x97Nimpxc3x97(Vdsxe2x88x92Veff+xcfx86o)))
Here, ∈s denotes a dielectric constant of silicon, q denotes an electric charge, Nimp denotes a density of impurities, Vds denotes an electric potential difference between the drain and the source, Veff denotes an effective voltage indicated by an electric potential difference between Vgs (an electric potential difference between gate and source) and Vth (a threshold voltage), and xcfx86o denotes a built-in potential. In case of ∈s=Ksxc3x97∈o =104 pF/m, Nimp =1xc3x971023 m xe2x88x923, Vds =2 V, Veff=Vgsxe2x88x92Vth=0.5 V and xcfx86o=0.9 V, xcex=0.15 Vxe2x88x921 is obtained. Therefore, Rin(CS)=133 xcexa9 is obtained.
Therefore, the input resistance of the current source circuit is considerably higher than the output resistance of the source follower (Rout(SF) less than Rin(CS)).
Because the conventional current driver circuit has the above-described configuration, the output impedance of the current driver Tr1 or Tr2 differs from the input impedance of the current source circuit. Therefore, it is difficult to obtain both the impedance matching at a connection point between the current driver Tr1 or Tr2 and the write head WH and the impedance matching at a connection point between the write head WH and the current source circuit. As a result, when data of the magnetic signal is written to the magnetic disk, xe2x80x9cundershootxe2x80x9d undesirably occurs, and a problem has arisen that data of the magnetic signal cannot be accurately written to the magnetic disk.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional current driver circuit, a current driver circuit in which data is accurately written at high data transfer rate.
The object is achieved by the provision of a current driver circuit which includes a first switching transistor of which one end is connected with a high electric potential power source and of which the other end is connected with a first end of a first resistor, a second switching transistor of which one end is connected with the high electric potential power source and of which the other end is connected with a first end of a second resistor, a third switching transistor of which one end is connected with a low electric potential power source and of which the other end is connected with the first end of the first resistor, a fourth switching transistor of which one end is connected with the low electric potential power source and of which the other end is connected with the first end of the second resistor, and an inductance element of which both ends are connected with a second end of the first resistor and a second end of the second resistor respectively.
Here, a current supply circuit includes a first group of the first switching transistor and the first resistor or a second group of the second switching transistor and the second resistor, and a current receiving circuit includes a first group of the second resistor and the fourth switching transistor or a second group of the first resistor and the third switching transistor. In this case, the inductance element, the current supply circuit and the current receiving circuit are set so as to match both an output impedance of the current supply circuit and an input impedance of the current receiving circuit with an impedance of the inductance element.
Accordingly, even though a current flowing though the inductance element is changed at high frequency to accurately write data to a magnetic storing device at high data transfer rate, the occurrence of xe2x80x9cundershootxe2x80x9d can be prevented, and data can be accurately written at high data transfer rate.